1. Field of the Invention.
The present invention relates generally to optical reflectors, more particularly to optical reflectors for solar energy systems, and specifically to a reflector assembly comprising a secondary optical element (SOE) for matching concentrated solar insolation forming a circular image to a square, photovoltaic receiver.
2. Description of the Related Art.
Solar insolation has several important advantages as an energy source. Unlike fossil fuels, it is renewable indefinitely. It is also regarded as "free" energy, as distinguished from fossil and nuclear fuels which are commonly sold for energy use. Solar energy has the further advantage of avoiding most of the environmental and toxic waste problems associated with energy production from fossil and nuclear fuels.
Most parts of the world receive far more solar energy than is required to meet local energy demands. However, effective utilization of the available solar insolation often requires converting it to a different energy form, such as thermal or electrical. Furthermore, since neither the availability of solar energy nor the general demand for energy are constant, for some applications it is desirable to convert solar energy to a form of energy that can be stored.
Photovoltaic energy systems convert light energy, e.g. solar insolation, to electrical energy. Semiconductors are commonly used to construct photovoltaic cells, which may be grouped in modules of cell matrices to provide a desired level of electrical power output. A photovoltaic receiver may comprise a number of individual photovoltaic cells, which are linked in both series and parallel combinations or circuit branches.
Photovoltaic systems can operate on direct solar insolation. However, because the photovoltaic collectors tend to be relatively expensive, it is sometimes more cost effective to concentrate the solar insolation which impinges upon the photovoltaic collector.
For example, photovoltaic systems have been designed with relatively high concentrations of five hundred times the ambient solar insolation level. Various types of reflective and refractive concentrators have heretofore been devised, including central receiver concentrating solar systems, dish concentrating solar systems, line focus concentrating solar systems and smaller lens point focus concentrating solar systems. A relatively cost effective concentrator may be designed with a flexible membrane formed in a dish-shaped configuration with an axis which is directed at the sun. The resulting flux concentration is cone-shaped, and the receiver may intercept the conical flux concentration on either side of its focal point, with the receiver surface preferably lying in a plane substantially normal to the axis.
However, the concentrated light image provided by a typical dish-shaped concentrator is circular in planform, whereas the optimum receiving surface planform for many photovoltaic receivers is square. Thus, photovoltaic energy system designers have been confronted with the problem of matching a square-surfaced photovoltaic receiver with a circular, concentrated light image. The receiver could be oversized to receive practically all of the concentrated light, but the corners of the receiver would then receive little or no concentrated light.
Photovoltaic cell operating efficiency is generally related to the illumination of the cell and the electrical current therethrough. A circuit branch of series-linked photovoltaic cells generally operates only as efficiently as the individual cell receiving the least amount of illumination. Hence a cell which does not receive adequate illumination can affect the performance of all other cells in series with it. Since the voltage output of the individual photovoltaic cells is often fairly low, photovoltaic receivers commonly include circuit branches of individual cells linked in series to provide a desired output voltage level. Therefore, optimum operating efficiency and cost effectiveness is generally achieved by providing substantially uniform flux density over the entire receiver surface.
Uniform flux density on the receiver surface could be achieved by oversizing the collector so that the receiver surface fits completely within its image. However, a problem with this solution is that a substantial amount of concentrated flux may miss the receiver completely because the flux concentration image would necessarily have a greater area than the receiver surface. Such an energy system would therefore have a lower operating efficiency corresponding to its concentrated flux losses.
Secondary optical elements (SOE's) have heretofore been proposed for improving the level of uniform flux density and improving the utilization of the concentrated flux. Such previous SOE's had various configurations, including circular, prismatic and pyramidal. Some previous SOE's had refractive optical elements, and others had reflective optical elements. Although these previous SOE's were capable of providing some degree of improvement in system efficiency, none had the advantages and features of the present invention.